Hammers drills are power tools which can operate in one of three modes of operation. Generally, a hammer drill will have a tool bit which can be operated in a hammering mode, a rotary mode and a combined hammering and rotary mode.
Hammer drills also generally comprise a mode change mechanism which enables a user to select between the different modes of operation of the hammer drill.
European patent application EP0759342 discloses a hammer drill having a mode change mechanism comprising an axially slidable lock ring which is disposed on the spindle of the hammer drill. The rotational mode of the hammer drill is selected by rotating an eccentric pin which moves the lock ring in the axial direction long the spindle in order to couple or decouple the lock ring from a tool holder to selectively cause rotation of the tool holder.
U.S. Pat. No. 5,456,324 discloses a hammer drill having a rotatable drive cylinder containing a hollow piston, the drive cylinder adapted to hold a tool bit such that the tool bit can be used in both a rotary mode and a reciprocating mode. A drive wheel is rotatably mounted on the drive cylinder, the drive wheel being geared to the motor of the tool. A coupling sleeve is key coupled to the drive cylinder so that the coupling sleeve can slide axially along the drive cylinder and also rotate with the drive cylinder. Both the coupling sleeve and the drive wheel have sets of teeth formed thereon such that they can intermesh. When the coupling sleeve is slid along the drive cylinder under the influence of a coil spring such that the teeth and the coupling sleeve and the teeth on the drive wheel intermesh, rotational motion is transmitted to the drive sleeve. The movement of the coupling sleeve along the spindle is accomplished by contact with an eccentrically mounted pin disposed on a rotating knob.
U.S. Pat. No. 5,379,848 comprises a hammer drill having a rotary drive sleeve comprising a tool holder, and an axially displaceable switching sleeve that can slide along the spindle in order to selectively couple the rotary drive sleeve to the rotational drive of a motor. The switching sleeve is biased into an operative position by a coil spring, and is moved by an eccentrically mounted pin.
U.S. Pat. No. 5,125,461 discloses a hammer drill having a stop element which in a first position permits axial displacement for the activation of the hammer mechanism, and a second position in which the stop element blocks the axial displacement, thus preventing the hammering action of the hammer drill.
U.S. Pat. No. 6,557,648 discloses a hammer drill having a motor with a rotary drive shaft, a housing accommodating the motor therein, and a mode change mechanism comprising a first gear with a claw portion and engaged with the drive shaft for transmitting rotation of the drive shaft, and a second gear having a claw portion and engaged with the drive shaft for transmitting rotation of the drive shaft. The mode change mechanism comprises a first drive sleeve having a claw portion enageable with the claw portion of the first gear for transmitting rotation of the drive shaft when the claw portion of the first sleeve is engaged with the claw portion of the first gear, a crank shaft driven in response to the rotation of the first drive sleeve, and a hammer mechanism responsive to the rotation of the reciprocating drive shaft for transmitting a reciprocating striking force to a tool bit. The mode change mechanism comprises a second drive sleeve having a claw portion enageable with the claw portion of the second gear for transmitting rotation of the drive shaft when the claw portion of the second sleeve is engaged with the claw portion of the second gear, a rotary drive shaft driven in response to the rotation of the second drive sleeve, and a rotary mechanism responsive to rotation of the rotary drive shaft for transmitting a rotational force to the tool bit. The mode change mechanism further comprises a switching mechanism for selectively engaging or disengaging the claw portion of the first drive sleeve with or from the claw portion of the first gear and also selectively engaging or disengaging the claw portion of the second drive sleeve with or from the claw portion of the second gear.
The switching mechanism includes a rotatable switching lever with two eccentric pins. One pin is for moving the first drive sleeve upwards and the other pin is for moving a shift member upwards so as to engage with, and move upwards, the second drive sleeve. The shift member is slideably mounted on a switch assist shaft substantially parallel to the crank shaft and rotary shaft. A spring biases the shift member downwards. This is in addition to the springs that bias the first and second drive sleeves downwards so that their claw portions engage the claw portions of the first and second gears, respectively. Therefore, the switching mechanism is a relatively complex system involving several moving parts which make it expensive to manufacture and assemble.
Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.